Crystal controlled oscillator



NOV. 29, 1949 RCYDEN 2,489,327

CRYSTAL CONTROLLED OSC ILLATOR Filed Sept. 14, 1945 INVENTOR GEORGE I. ROYDEN 28 ATTORNEY Patented Nov. 29, 1949 CRYSTAL CONTROLLED OSCILLATOR George T. Royden,

South Orange, N. J., assignor to Federal Telephone and Radio Corporation,

New York, N. Y.,

a corporation of Delaware Application September 14, 1945, Serial No. 616,343

11 Claims. 1

This invention relates to electric oscillators, and more in particular to an oscillator system where the frequenc of oscillation is controlled solely by a crystal.

An object of this invention is to provide a source of high-frequency electrical current wherein the frequency is controlled solely by a band-pass filter such as a crystal. A further object is to provide an oscillator system which will operate over a wide range of frequencies without the necessity of adjusting components such as conductances and capacitors, but which will maintain oscillation at any particular desired frequency. A further object is to provide an oscillator circuit which is simple in construction, efficient in operation and which can be readily adapted to different uses. A still further object is to provide an oscillator circuit which includes a band-pass filter such as a crystal which will pass only a particular desired frequency, and wherein the frequency of oscillation is controlled solely by the band-pass filter.

The invention accordingly consists in the features of construction, combinations of elements, arrangements of parts and in the several steps and relation and order of each of the same to one or more of the others, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claims.

In the drawing:

Figure l is a circuit diagram representing one embodiment of the invention; and

Figures 2 and 3 are similar to Figure 1, but represent other embodiments of the invention.

Under certain circumstances it is desirable to provide an oscillator circuit which is sturdy and simple in construction but which will maintain a frequency of oscillation at any particular frequency desired. Such systems have been proposed, but generally when it is desirable to operate at various frequencies over a range it has been necessary to adjust two or more of the various components of the circuit. In accordance with the present invention such an oscillator is provided wherein adjustment of the components is unnecessary. In the illustrative embodiments of the invention the frequency of oscillation is determined solely by the frequency which will be passed by a crystal the frequency of which is inherent in it. Thus, for any frequency within the range for which the circuit is built the frequency of oscilaltion may be initially determined, or later changed, by merely inserting a crystal having a particular desired series resonant frequency.

Referring to Figure 1 of the drawing, a tube l has elements forming two triodes 2 and 3 which have a common cathode 4, separate control grids 5 and 6, respectively, and separate plates 1 and 8, respectively. In this tube, as well as in the tubes of the other embodiments, the cathodeheater circuits are omitted. The circuit is energized by a suitable source of D. C. voltage connected between a negative terminal 9 and a positive terminal l0. Cathode4isconnectedtoterminal 9 through a bias resistor ll, while plate 1 is connected directly to terminal Ill, and plate 8 is connected to terminal l0 through a high-frequency choke coil [2. A grid resistor 13 connects grid 5 to terminal 9 and grid 6 is connected directly to terminal 9. Plate 8 is connected to grid. 5 through a piezoelectric quartz crystal M which is used as a band-pass filter to pass current only at the series resonant frequency of the crystal; this is the frequency at which it is desired that the circuit oscillate.

For purposes of explaining the operation of this circuit, assume that there is a slight rise in the potential of grid 5; this will cause an increase in the flow of current through triode 2 with the result that there is an increase in the current flow through bias resistor II. The rise in current flow through bias resistor ll causes cathode t to be at a potential which is higher than that of terminal 9 and grid 6. Thus, the rise in the potential of grid 5 causes a decrease in the bias potential of grid 6 so that there is a decrease in the current flow in triode 3. This decrease of current flow in triode 3 causes a rise in the potential of plate 8 and this rise in potential is fed back through crystal H to grid 5. The components of the circuit are so chosen that any instantaneous increase in the flow of current through triode 2 is greater than the resulting instantaneous decrease in the flow of current through triode 3. Thus, an increase in the current flow through triode 2 always causes an increase in the potential drop across resistor H, and, although this increase is neutralized in part by the resulting instantaneous decrease in the flow of cur rent through triode 3, the net result is an increase in the potential change 01'' grid 5 of triode 2.

In a similar manner, any instantaneous decrease in the potential of grid 5 is followed by a corresponding instantaneous decrease in the flow of current through triode 2 and a resulting increase in the flow of current through triode 3, so that plate 8 becomes more negative; and, this negative change is fed back through crystal I4 to grid 5. However, the instantaneous decrease in the flow of current through triode 2 is greater than the instantaneous increase in the flow of current through triode 3 so that the net result is a decrease in the potential of grid 5.

It is thus seen that any instantaneous rise or fall in the potential of grid causes an accentuated rise or fall in this potential so that the swing of the grid potential of grid 5 is increased. This condition tends to set up oscillations in the circult; and, because crystal l4 passes current only at its series resonant frequency, the circuit will oscillate at this frequency. The oscillator output is taken through a coupling condenser l5 and a terminal I6.

It is thus seen that an oscillator circuit is provided which constitutes a source of A. C. potential, the frequency of which is controlled solely by crystal 14. The range of frequencies at which a particular circuit will operate under the control of various crystals is very wide, although there are practical limits to the range of frequencies for any particular circuit, depending of course, upon the particular components of the circuit. In Figure 1 the two triodes 2 and 3 are enclosed in a single envelope and thus are designated as a single tube I whereas they are the equivalent of two tubes.

In the embodiment of Figure 2 a system is provided for keying the output from the oscillator, and two separate tubes are shown. As shown at the left of the figure, a tube I! has a cathode IS, a plate I9, a control grid 23, a screen grid 2i and a suppressor grid 22. At the right, a tube 23 has a cathode 24, a grid 25, and a plate 26. Cathodes l8 and 24 of the two tubes are connected together and thence are connected through a bias resistor 2'! to the negative terminal 28 of a suitable source of direct current voltage. Plates l9 and 26 are connected respectively through high-frequency choke coils 29 and 30 to the positive terminal 3| of this source of power. Grid 20 is connected to the negative terminal 23 through a grid resistor 32, and grid 25 is connected directly to terminal 28. Plate 26 is connected to grid 20 through a piezoelectric quartz crystal 33 which is similar in its characteristics and function to crystal M of Figure 1. Screen grid 2| is connected to the positive terminal 3| through a resistor 34 which is by-passed to the negative terminal 28 by a condenser 35. Suppressor grid 22 is connected through a resistor 36 to a source of negative potential at terminal 3'! which is suificient to reduce substantially or cut off the flow of current to plate l9. Suppressor grid 22 also may be connected to the negative terminal 28 by the closing of a key 38, and, with this key closed, the current flow to plate I9 is not suppressed.

The operation in the circuit of Figure 2 is similar to that in the circuit of Figure 1. For

example, assuming that there is a rise in the potential of control grid 20 of tube N, there will be an increase in the flow of current through this tube which will result in an increase in the flow of current through bias resistor 21. This will cause the potential of cathode l8 to rise with respect to terminal 28 and grid 25 of tube 23. Cathode 24 of tube 23 is at the same potential as cathode l8 and therefore grid 25 will become more negative with respect to its cathode 24 with the result that there is a reduction in the current flow through tube 23 and, therefore, an increase in the potential of plate 26. At the series resonant frequency of crystal 33 any change in the potential of plate 26 is fed through crystal 33 to grid 20 of tube 11. Here, as in the embodiment of Figure 1, the components of the circuit are so chosen that any instantaneous change in the flow of current through tube I1 is greater than the resulting instantaneous change in the flow of current through tube 23 so that there is a net effect of an increase in the swing of the grid potential on grid 20 of tube l1. Therefore, the conditions exist which are necessary for oscillation in the circuit at this resonant frequency, and the circuit oscillates. The resultant A. C. output is taken from the circuit through a coupling condenser 39 and a terminal 40.

The above-described mode of operation is on the basis that suppressor grid 22 is not interfering with the fiow of current through tube I1 and this is the true condition when key 38 is closed. However, when key 38 is open the current flow through tube I! is substantially cut-off so that there is no appreciable output at terminal 40. Thus, key 38 controls the A. C. output and by operating key 38 the circuit may be 'used for telegraph operation. When desirable, suppressor grid 22 may be connected to ground through the secondary of a transformer on which audio-frequency signals are impressed, thus, to control the voltage of the suppressor grid. Under these conditions the A. C. output at terminal 43, which is at radio-frequency, is modulated by the impressed audio-frequency signals and this modulated output is delivered to an antenna.

Figure 3 represents an embodiment of the invention wherein an oscillator of the same construction and operation as that of Figure 2 is provided additionally with an arrangement for automatically regulating the amplitude of oscillation by means of a diode element in tube 23. Accordingly, a pair of resistors 4| and 42 of relatively high value are joined at a juncture 43 and are connected between grid 2| of tube Ill and the negative terminal 28 of the source of power. This juncture 53 is connected at a juncture 43 through grid resistor 32 to grid 20 of tube IT and also to an anode :35 in tube 23, which anode forms with cathode 24 a diode rectifier element. Thus, re sistors 34, 4t and 42 act as a voltage divider and tend to maintain the juncture 43, and therefore grid 23, at a predetermined voltage. A condenser 46 is connected between juncture 44 and negative terminal 28, and is normally charged to the potential of juncture 43.

During operation, oscillation occurs because a rise in potential of grid 20 causes an increase in the fiow of current through tube 11, and a decrease in the flow of current through tube 23, with the result that the potential of plate 26 of tube 23 in creases; and, this increase in potential is fed back through crystal 33 to grid 20. If the amplitude of oscillation rises to such an extent that the negative peak voltage on cathode 24 becomes more negative than the potential of anode 45, as determined by the voltage of juncture 43 of the voltage divider, current will flow through the diode causing an increase in the current flow through resistors 34 and 4|, and this causes a decrease in the potential of juncture 43 and therefore of grid 23. This decrease in potential is averaged by condenser 43 and reduces the gain of tube II. This action tends to adjust the grid bias on grid 23 and thus the gain of tube H to a value just sufiicient to maintain the desired magnitude of oscillation. This amplification control action is particularly effective where the tube is of the type having a variable amplification factor.

In the embodiment of Figure 3 the components of the circuit and the voltages are so chosen as to obtain class A operation of the vacuum tubes. In all of the illustrative embodiments of the invention, the radio-frequency output may be taken from the common cathode circuit, when desirable, or from any other point in the circuit at which the radio-frequency potential appears.

As many possible embodiments may be made of the mechanical features of the above invention and as the art herein described might be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinabove set forth, or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. In an oscillator, the combination of an oscillator tube having grid and plate electrodes, a control tube having grid and plate electrodes, a coupling connection between a plate electrode of said oscillator tube and a grid electrode of said control tube, a positive feedback connection between a plate electrode of said control tube and a grid electrode of said oscillator tube, said feedback connection including a band-pass filter adapted to sustain oscillations of predetermined frequency in said tubes, and means for limiting the ampli tude of said oscillations, said means including a connection between a plate electrode of said control tube and a grid electrode of said oscillator tube for applying a corrective voltage to the latter electrode, rectifier means in the last-named connection, and means for biasing said rectifier means so as to block application of said corrective voltage except upon occurrence of amplitudes exceeding a predetermined limit.

2. In an oscillator, the combination set forth in claim 1 wherein said band-pass filter is represented by a piezo-electric crystal.

3. In an oscillator, the combination set forth in claim 1 wherein said rectifier means comprises a diode anode placed in a common envelope with the electrodes of said control tube.

4. In an oscillator, the combination set forth in claim 3 wherein said diode anode is connected to a condenser shunting the said rectifier means.

5. In an oscillator, the combination set forth in claim 4 wherein the said biasing means is represented by a voltage divider comprising fixed resistors connected between a positive and a negative terminal of a source of direct current potential, an intermediate point on said voltage divider being connected to said diode anode.

6. An electronic oscillator comprising a first vacuum tube having a cathode, a plate and at least one grid, a second vacuum tube having a cathode, a plate and a grid, a coupling connection between said two tubes including a resistor connected at a common point to both of said cathodes, a positive feedback connection between the plate of said second tube and a grid of said first tube, said feedback connection including a bandpass filter adapted to sustain oscillations of predetermined frequency in said tubes, and means for limiting the amplitude of said oscillations, said means including a diode anode placed inside the second tube adjacent the cathode thereof, a connection between said diode anode and the said grid of the first tube, and means for biasing said diode anode to a potential lower than that of the associated cathode except upon occurrence of amplitudes exceeding a predetermined limit.

'7. An electronic oscillator according to claim 6, in combination with means for applying a var' able biasing potential to a further grid of said first tube, whereby the magnitude of said oscillations may be controlled.

8. An electronic oscillator according to claim 7, wherein said means for applying a variable potential comprise a plurality of sources of difierent fixed potentials including ground, a permanent connection between said further grid and one of said sources other than ground, a resistor in said permanent connection, and manually operable switch means for grounding the last-named source over said resistor.

9. An electronic oscillator according to claim 6, further comprising a condenser shunting said diode anode, the cathode associated therewith, and the said resistor, said condenser being connected between said diode anode and ground.

10. An electronic oscillator according to claim 9, further comprising a source of positive voltage, a plurality of resistors connected between said source and ground, and a connection between said diode anode and the juncture of two of the last-mentioned resistors.

11. An electronic oscillator according to claim 6, wherein said band-pass filter is represented by a piezo-electric crystal.

GEORGE T. ROYDEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,606,791 Horton Nov. 16, 1926 2,269,417 Crosby Jan. 6, 1942 2,300,996 Vanderlyn Nov. 3, 1942 2,303,862 Peterson Dec. 1, 1942 

